Jim Fitzgerald wrote: Why do we need another device when the US has mandated ADS-B?
FYI: Flarm was developed in Europe, and I understand that it is popular there. A version of Flarm called Powerflarm is used here in the USA by the glider community, who decided a few years back that they couldn't wait for ADS-B implementation. The Flarm part of Powerflarm requires no ground network, but works only among equipped aircraft. Powerflarm, the USA version of Flarm, also includes PCAS and an ADS-B receiver in the same box.
Glider operations represent a special problem for conflict detection/collision prevention because gliders 1) Turn and circle often, and 2) Often share a thermal, so they frequently end up maneuvering close to each other. Flarm was specifically developed to deal with these odd flight paths, and can extrapolate ahead to predict when they might result in a conflict.